$(function(){
  $('img[usemap]').each(function(){
    var $this = $(this),
        originalWidth = $this.attr('width')*4,
        width = $this.width(),
        $map = $($this.attr('usemap'));
    $map.find('area').each(function(){
      var $area = $(this),
          coords = $area.attr('coords').split(',');
      $.each(coords,function(i){
        coords[i] = coords[i] * width / originalWidth;
      });
      $area.attr('coords',coords.join(','));
    });
  });
});